In a radio communication network, the location of a user equipment terminal is stored in the network on a tracking area basis. The tracking area corresponds to a location area (LA) or a routing area (RA) in the third generation (3G) system and includes one or more cells.
An identifier of an access gateway apparatus for managing each user equipment terminal is maintained by a home location register (HLR).
A user equipment terminal has two operational modes; one is an active mode and the other is an idle mode.
The user equipment terminal in the idle mode (i.e., in a standby status) selects a cell with the best propagation condition and receives a paging channel transmitted in the cell. After the user equipment terminal checks the received paging channel, determines the presence or absence of a flag indicating an incoming call to the user equipment terminal, and identifies the flag indicating the incoming call to the user equipment terminal, the user equipment terminal accesses the cell within which the user equipment terminal is situated in uplink. Then, the user equipment terminal establishes a connection in the cell and changes to the active mode. At this moment, the network (network nodes) can identify the cell within which the user equipment terminal is situated.
Since there is no connection between the network (network node) and the user equipment terminal while the user equipment terminal is in the idle mode, the network (network nodes) cannot identify the cell within which the user equipment terminal is situated. For this reason, the network (network nodes) transmits the paging channel in all the cells included in the tracking area.
In each tracking area, an identifier of the tracking area is broadcast in each cell. The user equipment terminal in the idle mode receives the broadcast identifier of the tracking area and recognizes the tracking area to which the user equipment terminal belongs.
For example, when the user equipment terminal moves to an adjacent cell and the identifier of the tracking area which is broadcast in the adjacent cell is different from the previously broadcast identifier of the tracking area, the user equipment terminal accesses the adjacent cell in uplink and requests an update of the tracking area. The update of the tracking area is referred to as a tracking area update (TAU).
Specifically, the user equipment terminal maintains the identifier of the tracking area with which the location of the user equipment terminal is associated. The user equipment terminal reselects a cell as it moves. Reselecting of the cell is referred to as cell reselection. Each time the user equipment terminal performs cell reselection, the user equipment terminal receives broadcast information in the cell and checks the identifier of the tracking area included in the broadcast information. When the received identifier of the tracking area is different from the maintained identifier of the tracking area, the user equipment terminal determines that the user equipment terminal has moved out of the tracking area within which the user equipment terminal has been situated, and then performs a tracking area update.
On the other hand, when the received identifier of the tracking area is the same as the maintained identifier of the tracking area, the user equipment terminal determines that the user equipment terminal has not moved out of the tracking area within which the user equipment terminal has been situated. In this case, the user equipment terminal does not perform a tracking area update.
Although the user equipment terminal performs cell reselection as it moves, the user equipment terminal does not perform a tracking area update as long as the user equipment terminal is situated within the same tracking area.
Further, the user equipment terminal in the active mode (i.e., the user equipment terminal in communication) does not perform a tracking area update.
As described above, the tracking area includes one or more cells. Among these cells, a set of cells included in the same band (frequency band) is referred to as a frequency layer. Each frequency layer includes one or more cells.
For example, as shown in FIG. 1, one or more cells are included in a tracking area. In the tracking area, frequency layers F1, F2, and F3 are provided. In each frequency layer, at least one carrier (cell) is provided. For example, in the frequency layer F1, two carriers (cells) f.A1 and f.A2 are provided. In the frequency layer F2, three carriers (cells) f.B1, f.B2, and f.B3 are provided. In the frequency layer F3, three carriers (cells) f.C1, f.C2, and f.C3 are provided. Different frequency bands are used in the respective frequency layers. Further, different radio access technologies (RATs) may also be used in the respective frequency layers. For example, the frequency layers F1 and F2 may correspond to the LTE system and the frequency layer F3 may correspond to the UMTS (Universal Mobile Telecommunications System).
The user equipment terminal is situated within one of the plural cells included in the frequency layers. The network (network node) can transmit priority information indicating the frequency layer to be used and specify a priority of the frequency layer in which the user equipment terminal should be situated. For example, when the priority of the frequency layer F1 is specified to be high, the user equipment terminal is situated in the frequency layer F1 as long as the user equipment terminal is within the coverage of the frequency layer F1.